STUDY OF STRATA BEHAVIOUR IN BLASTING GALLERY PANEL IN COAL MINES A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF BACHELOR OF TECHNOLOGY IN MINING ENGINEERING BY B.N.V. SIVA PRASAD 109MN0505 DEPARTMENT OF MINING ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA – 769008
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1
STUDY OF STRATA BEHAVIOUR IN BLASTING
GALLERY PANEL IN COAL MINES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MINING ENGINEERING
BY
B.N.V. SIVA PRASAD
109MN0505
DEPARTMENT OF MINING ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA – 769008
1
STUDY OF STRATA BEHAVIOUR IN BLASTING
GALLERY PANEL IN COAL MINES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MINING ENGINEERING
By
B.N.V. SIVA PRASAD
109MN0505
UNDER THE GUIDANCE OF
PROF. D.P. TRIPATHY
PROF. S. JAYANTHU
DEPARTMENT OF MINING ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA – 769008
ii
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA
CERTIFICATE
This is to certify that the thesis entitled, “Study of Strata Behaviour in Blasting Gallery
Panel in Coal Mines” submitted for the award of the degree of Bachelor of Technology
(Mining Engineering) in National Institute of Technology (NIT) Rourkela, is a record of
original research work carried out by Sri B.N.V. Siva Prasad under our supervision. The
context of this thesis has not been submitted elsewhere for the award of any degree to the best
of my knowledge.
(Dr. Debi Prasad Tripathy)
Professor
Department of Mining Engineering
NIT Rourkela
Date :
(Dr. Singam Jayanthu)
Professor
Department of Mining Engineering
NIT Rourkela
Date :
iii
ACKNOWLEDGEMENT
Sincere thanks and deep respect to my guides, Dr. Debi Prasad Tripathy and Dr. Singam
Jayanthu, Professors, Department of Mining Engineering, National Institute of Technology,
Rourkela for their valuable suggestions, focused guidance and continuous help with moral
support to complete the thesis within the stipulated time frame.
I am thankful to the management of the Singareni Collieries Co. Ltd., AP, for allowing me to
collect the requisite information/data for the purpose of my research work. I am also thankful
to the mine managers, safety officers and other staffs of SCCL especially of GDK-10 incline,
RG-III area for providing necessary permission and data on Blasting Gallery method and also
sharing their valued experience on BG working which has undoubtedly improved the quality
of this thesis.
My special thanks to the authority of National Institute of Technology, Rourkela for
rendering permission to use their most modern and updated library in connection with the
literature survey of the Project work.
Date :
B.N.V. Siva Prasad
Department of Mining Engineering
National Institute of Technology
Rourkela – 769008
iv
CONTENTS
ITEMS TOPIC PAGE
NO.
A Abstract Vi
B List of Tables viii
C List of Figures Ix
CHAPTER TITLE PAGE
NO.
1 INTRODUCTION 1
1.1 Problems Of Strata Behaviour Characteristics With
Respect To Indian Coal Seams
3
1.2 Objectives of the Project 3
2 LITERATURE REVIEW 4
2.1 Blasting Gallery Method 5
2.2 Strata Behaviour 6
2.3 Theories of Mechanics of Strata Behaviour 6
2.4 Strata Pressure Redistribution in Bord & Pillar Workings 6
2.5 Convergence in Blasting Gallery Workings 7
2.6 Indian Scenario 7
2.7 International Scenario 9
3 NUMERICAL MODELLING 11
3.1 Overview 12
3.2 Problem Solving With FLAC 13
3.3 Recommended Steps For Numerical Analysis In
Geomechanics
13
3.4 Steps in Numerical Modelling 13
4 METHODOLOGY 17
4.1 Selection Of Sites 18
4.2 Instrumentation 18
4.3 Modelling parameters 21
4.4 Sequence of Modelling 21
v
4.5 Experimentation 22
4.5.1 Sample Preparation 22
4.5.2 Triaxial Testing 23
5 FIELD INVESTIGATION 28
5.1 Strata Behaviour Observations in BG Panel 3A, GDK 10
Incline, SCCL
29
6 RESULTS & DISCUSSIONS 48
6.1 Numerical Modelling Outputs : Convergence 49
7 VALIDATION OF MODEL 54
7.1 Comparison of Modeling Results with Field
Investigation Data
55
8 CONCLUSIONS & SUGGESTIONS 57
8.1 Conclusions 58
8.2 Suggestions 58
8.3 Limitations 59
9 REFERENCES 60
APPENDIX 63
Appendix I - Details of the Panel 64
Geo-Mining Conditions Of The Panel – 3A 64
Support system in the BG-3A panel 66
Additional support 67
Measures against strata control problems 67
Measures adopted to prevent strata control problems 67
Appendix II – Numerical Modelling Code of BG 3A
PANEL, GDK 10 Incline, SCCL
68
vi
ABSTRACT
In mining industry, the challenging task of a mining professional comprises of the extraction
of maximum natural resources with utmost safety of the miners. This task becomes more
problematic when the thickness of coal seam is larger. “Blasting Gallery” method is a unique
technique of depillaring thick seams for higher recovery of coal. The extensive literature
survey clearly helps to understand that the ultimate potentiality of the method is yet to be
explored. Though a number of researchers, academicians and other stake holders attempted to
work on it but impact of many significant parameters are still to be analyzed.
The Blasting Gallery operation in a mechanized underground mine system depends upon
many decisions influenced by the geo-technical parameters which are often interspersed with
inherent strata configurations. The present study has been aimed to examine thoroughly BG
method operational systems in Indian geo-mining conditions such as:
Study of roof convergence with respect to face advancement during different
stages of extraction of coal in Blasting Gallery panels in SCCL mines.
Simulation of field conditions in the numerical model generated using FLAC.
Interpretation of strata behaviour through numerical modeling using FLAC.
In order to study the strata behaviour of such coal mines with thick seams, GDK 10 incline,
3A panel of Singareni Collieries Company Limited (SCCL), Ramagundam was selected. This
mine has a thick coal seam of 11m and is at depth of 350mtr, practicing Blasting Gallery
method to the maximum extent. Convergence behaviour with respect to goaf edge distance
(GED) was monitored with the help of high state-of-the-art equipment (calibrated)
throughout the life of BG panel. An over emphasis was given on the field study where data of
BG mine specially related to natural falls, induce blasting etc. were recorded. Convergence of
roof strata in mm, corresponding goaf edge distance (GED) in meter (m), corresponding
distance from face in meter (m) and depth of panel in meter (m) were measured to know the
significant impact of different layers with varying overburden pressure which leads us to
think some logical sequence of interrelated operations.
vii
The coal sample was collected from the mine and was tested for determination of the rock
mass parameters. The geo-technical conditions of the mine were simulated and Numerical
Modelling was carried out by using the most sophisticated software – FLAC. The output
results obtained from the mine data was compared with that of model data and distance from
goaf edge was considered as a sensitive variable so that the validation would represent the
system in totality. The different conclusions drawn from this work is enumerated as follows:
The maximum rate of convergence and cumulative convergence recorded in field was
about 4mm/day and 61mm respectively, measured at convergence station C-5 in 68
Level.
From the triaxial testing, the major principal stresses of 22, 32 and 41.5 MPa were
obtained at confining stresses of 0, 2 and 3 MPa respectively.
The results obtained from the RocLab software indicated the Cohesion, Friction
Angle, UCS and Tensile Strength values to be 1.1MPa, 30.840, 1.314 MPa and
0.32MPa respectively.
The model predicted maximum cumulative convergence to be 70mm while that
observed in field is 61mm.
The results obtained by FLAC when compared with that of the Field data, the
predicted value were within an approximation of 10% for stages I, III, IV & V
whereas for stages VI and VII, its in 20% approximation except for that of Stage II
which showed a higher value of cumulative convergence measurements due to
occurrence of natural fall.
viii
LIST OF TABLES
NO. Table PAGE
NO.
4.1 Results of Triaxial Test 26
4.2 Properties of Coal 27
5.1 Convergence Observation up to the end of the Month of June 11 31
5.2 Convergence Observation Up To the End of the Month of July 11 32
5.3 Convergence Observation Up to the End Of The Month Of August 11 33
5.4 Convergence Observation Up to the End Of The Month Of September11 34
5.5 Convergence Observation Up to the End of the Month of October 11 35
5.6 Convergence Observation Up to the End of the Month Of November 11 36
5.7 Convergence Observation Up to the End of the Month of December 11 37
5.8 Convergence Observation Up to the End of the Month of January 12 38
6.1 Maximum deformation of roof in advance workings (Galleries and Splits)
for various stages of extraction in BG panel in the numerical models 53
7.1 Comparison of FLAC Results & Field Investigation Data 56
10.1 Details of working BG Panel No. 3A of No. 3 Seam, Block – C 64
ix
LIST OF FIGURES
NO. TABLE PAGE NO.
2.1 Blasting Gallery Method 6
3.1 A general flow sheet of modelling procedure 16
4.1 Typical Instrumentation Set Up for Strata Behaviour Study 19
4.2 Convergence Indicator 19
4.3 In Situ Measurement Of Convergence 20
4.4 Instrumentation Layout in BG 3A Panel of GDK 10 Incline, SCCL 20
4.5 Coal Core Sample for Triaxial Testing 23
4.6 Laboratory Setup for Triaxial Testing 24
4.7 Enclosure of Triaxial Specimen 25
4.8 Triaxial Specimen in Pressure Chamber 25
4.9 Stresses Acting on Triaxial Specimen 25
4.10 Gage Length for Measurement of Strain on Triaxial Specimen 25
4.11 Deformation of Triaxial Specimen Under Load 26
4.12 Deviator vs. Confining Stress 26
4.13 Triaxial Reuslts using RocLab software 27
5.1 Convergence observations in level 66A 39
5.2 Convergence observations in Level 66B 40
5.3 Convergence observations in Level 67A 41
5.4 Convergence Observations in Level 67 B 42
5.5 Convergence observations in Level 67 43
5.6 Convergence observations in Level 68A 44
5.7 Convergence observations in Level 68B 45
5.8 Convergence observations in Level 68 46
5.9 Convergence observations in 70L 47
6.1 Maximum Deformation of Roof after Development of Pillars 49
6.2 Maximum Deformation of Roof after Splitting of Pillars 50
6.3 Maximum Deformation of Roof after Extraction of 1 Stook 50
6.4 Maximum Deformation of Roof after Extraction of 2 Stooks 51
6.5 Maximum Deformation of Roof after Extraction of 3 Stooks 51
x
6.6 Maximum Deformation of Roof after Extraction of 4 Stooks 52
6.7 Maximum Deformation of Roof after Extraction of 5 Stooks 52
6.8 Grid Generation in Final Stage 53
7.1 Convergence Results: FLAC Results vs. Field Investigation Data 55
A1 BG Panel #3A, GDK 10 Incline, SCCL Layout 65
A2 Borehole Section of BH No. 441 At GDK 10 Incline, SCCL 66
1
CHAPTER 1
INTRODUCTION
2
1. INTRODUCTION
Strata control or roof control implies the control of the strata to facilitate mining operation to
be done efficiently and safely. Not only we are concerned with the roof falls and uncontrolled
failure of strata or structure in the rock but also with the harnessing of the strata pressure to
advantage so that there is ease in coal getting. There is less emission of gas and less
production of dust and also the caved strata fills the goaf solid so that the risk of spontaneous
heating is minimized. Obviously, the strata on the face, and in the adjoining area, i.e. in front
and behind, must require attention so that no uncontrolled failure of the ground takes place.
In order to design satisfactory strata control measures it is essential first to have a clear
understanding of the mechanics of the movement of the ground as a result of mining
operation.
In thick coal seams, coal bed forms the roof of the lower slices. A coal layer at the roof
normally forms a good roof. But coals with joints and cleats are prone to fail without
warning. Some seams have coal balls, nodules or rounded fragments, and these may fall
unnoticed and cause fatalities. When coal bed is undermined, it may also develop induced
cleavages and fractures and in such situations roof falls are common. So for complete or
maximum extraction of such coal seams, the Blasting Gallery method is introduced.
First Blasting Gallery method of extraction was introduced in SCCL in 1989 at GDK No.10
Incline and being worked successfully. Although, first BG in India was introduced in East
Katras Colliery of Jharia Coal Fields, BCCL and Chora Colliery of Raniganj Coal Fields,
ECL in 1987, the workings were abandoned in East Katras Colliery due to Strata Control
Problem, and were discontinued in Chora Colliery due to premature Spontaneous heating
problem. GDK-10 Incline mine falls in Godavari Valley Coal Fields of Singareni Collieries
Company Limited and is situated in Andhra Pradesh.
This technology has become successful and popular at some mines of The Singareni
Collieries Company Limited, Andhra Pradesh and Chirimiri colliery of South Eastern
Coalfields Ltd, India. Thick seam ranging from 7 to 15 m is developed and a panel is found
suitable to extract within incubation period. Diagonal line of extraction is followed in
sequence to extract total thickness of coal by ring holes drilling and blasting and by using of
remote control load haul dumpers (LHD). This method of pillar extraction in a rib-less
method does not require any goaf edge support.
3
1.1 Problems of Strata Behaviour Characteristics with Respect to Indian Coal Seams
The BG working is not suitable for higher degree of gassiness with irregular seam
characteristics. BG method if applied in irregular seam will cause unblasted waste rock
mixture with the coal. Overriding of galleries may be a regular phenomenon if a attention in
not paid on the extraction pattern in time. Chances of air blast with consequent possibilities of
spontaneous heating in the goaf seem to be a major problem in this working. The increasing
roof pressure creates major difficulties with setting of goaf edge breaker line support. This
leads to more chances of coal losses with a significant reduction in overall performance of the
mine.
1.2 Objectives of the Project
The Blasting Gallery operation in a mechanized underground mine system depends upon
many decisions influenced by the geo-technical parameters which are often interspersed with
inherent strata configurations. The present study has been aimed to examine thoroughly BG
method operational systems in Indian geo-mining conditions such as:
Study of roof convergence with respect to face advancement during different
stages of extraction of coal in Blasting Gallery panels in SCCL mines.
Simulation of field conditions in the numerical model generated using FLAC.
Interpretation of strata behaviour through numerical modeling using FLAC.
4
CHAPTER 2
LITERATURE REVIEW
5
2. LITERATURE REVIEW
Blasting Gallery technology, the successful and popular method of extraction with a given set
of input, has been a good source of underground production in India. The moderate to high
overburden always poses a problem to tackle with the strata in day to day’s work. The
extensive literature survey has been a prime part of this system and has given a priority out of
all subroutines considered here for this purpose. Blasting gallery method is a unique
technique successfully developed in France, where it has been practiced in virgin thick seams
in Carmaux colliery. Blasting gallery method was earlier experienced at East Katras colliery
of the Bharat Coking Coal Limited and Chora colliery of the Eastern Coalfields Limited of
the coal India limited. Overriding at East Katras colliery and loss of supports as well as coal
had put some question marks on its further application in Indian coal Industry. But this
technology has become successful and popular at some mines of The Singareni Collieries
Company Limited, Andhra Pradesh and Chirimiri colliery of South Eastern Coalfields Ltd,
India. (Singh R.D. 1998)
2.1 Blasting Gallery Method
In this method a seam is developed into panels of about 100 m x 50 m. From the main
headings rooms are driven to the full width of the panel and the coal between the rooms is
blasted down to the full thickness of the seam and loaded by remotely controlled loaders. The
layout of a panel for working by Blasting Gallery method is shown in Figure 2.1. The life of
the rooms should be kept as short as possible so that they do not undergo excessive
convergence and the movement of the vehicles is not rendered any difficulty.
The advantage of this system of mining is that, it makes it possible to win narrow panels or
larger panels in which the seam conditions are unsuitable tor a longwall face. It does not
require highly experienced workers as a longwall face with 'Soutirage' working .It requires
substantially less investments than those required for a longwall with soutirage working and
the equipment required i.e., heading machines or jumbos and LHD can be easily transferred
to other roadways if the method is unsuccessful. Thick seams up to 15 m in thickness can be
extracted in one pass with percentage extraction ranging from 65 to 85%.The method is
highly flexible in that in a district with several units in operation, even if one of the units is
under breakdown, production from the district will continue to come. The time required for
preparation of a panel in relation to the total life of the panel if less than with other
mechanized methods. (Majumdar S et al. 2011)
6
Figure 2.1 Blasting Gallery Method (Jayanthu S. 2005)
2.2 Strata Behaviour
Strata control or roof control implies the control of strata to facilitate mining operations to be
done efficiently and safely. The strata in the face and in the adjoining area require attention
so that no uncontrolled failure of the ground occurs.
2.3 Theories of Mechanics of Strata Behaviour
The various common theories in this regard include the following:
Dome or Arch Theory
Beam or Plate Theory
Soil Mechanics Theory
Pseudo Plastic Theory
Hypothesis Based on Law of Deformation
Dynamic Rock Pressure Theory
2.4 Strata Pressure Redistribution in Blasting Gallery Workings
The development of pressure on pillars appears to be dependent on three factors, viz. Depth
from surface, Area of development, Ratio of areas of bord centers to area of pillar formed.
Full pressure on pillars due to the weight of overlying strata is experienced much sooner in
shallow mines than in deep mines. Research from SA report that the maximum pressure is
exerted about the center of the area developed, but is less on completed pillars, from this
point towards the direction of the solid undeveloped coal. The degree of pressure at the center
7
of any solid any developed area, nearly circular in plan, is approximately represented by thick
strata above the seam equal to radius multiplied by ratio of area of pillars to that of the bord
centers. Where the completed pillars in development are circular, the pressure operating
within that area may be likened in a diagram to a cone or parabola. If the developing are is
square in plan, the pressure is likened to be a pyramid.
2.5 Convergence in Blasting Gallery Workings
Convergence in development headings is influenced by the nature of roof and floor and width
of headings. Weak roof and floors and wide galleries gives more convergence than that with
strong roof and floor and narrow galleries. In Indian coal mines, as the immediate roofs are
generally strong, hardly any convergence is observed. In depillaring and sequence of
extraction, besides the nature of roof and floor and whether the goaf is caved or stowed.
Because of the stooks and remnants left, the convergence does not follow any predictable
pattern. When the goaf is stowed, the convergence is less than what would occur if the goaf is
caved.
2.6 Indian Scenario
Banerjee (2006) in his paper stated that Blasting Gallery method has been a popular method
in SCCL and 800-to 1000 tpd production has been obtained from many panels at a much
lower investment than in PSLW faces.
Jayanthu (2005) has given his view that in caving panels, the nether roof strata in goaf before
first major fall behaves as a simply supported beam fixed one end to the panel barrier with
the goaf edge support/rib adjoining the working face acting as other end. In view of extensive
qualitative observations and theory, the roof strata fail due to tensile fractures, while the sides
suffer with shear failures. Hence, using been analogy, critical span for a competent layer
clamped all around under uniformly distributed load can be derived on the basis of tensile
strength of the competent layer. However, the end conditions of clamped beam with change
to an edge supported beam due to development of tensely cracks on the upper surface near
the ends of the beam.
Ray, Singh and Banerjee (2005) assessed that the direction and magnitude of horizontal stress
has a significant influence upon caving of the rock mass. Horizontal stress impedes the
failure of rock mass as it provides a confinement to the rock mass subjected to underground
loading condition. Vertical stresses add to the development of tensile and sheer stresses. In
8
deeper workings where the vertical stress in considerably high, the roof cavability is better
compared to a shallow working having similar roof lithology.
Satyanarayana et al (2005) expressed their views that with decrease in the distance of the
monitoring point from the goaf edge, convergence is increased. This convergence attains its
maximum value at the goaf edge. Similar phenomena also happen for strata load.
Venkatanarayana (2004) stated that normally the goaf of long wall or of BG panel is
completely packed in the middle of the goaf and along the barrier goaf consolidation will be
less. Panel size had been reduced from 25,000 m2 to 16,000 m2. To induce the main fall
several induced basting were under taken in the panel. He found occurrences of several major
falls before closing of the panel.
While monitoring of strata movement during underground mining of coal, Rajendra Singh et,
al (2004) found that the value of mining induced stress over pillar and roof to floor
convergence during depillaring, generally, increases with decrease in distance of the
observation station with respect to the line of extraction. Similarly the values of other
parameters like bed separation, load on support etc. were also influenced by the face advance
of a depillaring panel.
Rajendra Singh et, al (2004) stated the most challenging job during implementation of the
blasting gallery method was to provide effective support to the high roof after wining of the
roof coal and they solved this problem simply by introducing cable bolting as a support
system for the high roof as well as for the overlying coal bed.
Jayanthu (2001) observed while investigating strata behavior in a Blasting Gallery panel
during extraction of bottom section pillars at greater depth that the rate of convergence
reached a maximum per value of about 3.5mm/ day during major roof fall in the panel.
Increasing rate of convergence may be attributed to the roof falls in the goaf associated with
about 60% to 80% filling of the goaf.
NIRM (2000) in their report of strata monitoring in BG panel at GDK-10 Incline described
that the area of extraction at the time of major roof falls was more than 12,000m2, without
any damage to the advance workings. Due to the influence of the barrier up to 25m alongside,
9
in general, induce blasting near the barrier may not contribute to the major roof fall. While
optimizing blast design and charge loading parameters in coal for ring hole blasting and in
stone for Induce blasting in degree –1 seam for Blasting Gallery method.
While Studying of weathering action on coal pillars and its effects on long term stability,
Biswas and Peng (1999) observed that if a coal pillar is exposed to moisture and if it has a
parting layer in it, structural deterioration takes place over time. This deterioration reduces
the load carrying capacity of the pillar.
During investigation in to the strata behavior of panel H in East Katras colliery, Raju et al
(1998) indicated the failure of the parting above junction of bottom section due to high tensile
abutment stresses and also suggested that the galleries of the two sections must be
superimposed and high support resistance is needed in junctions in top and bottom sections.
Samantha (1997) described in his paper that at Chora 10-pit colliery during working by
Blasting Gallery method the immediate roof was very brittle and quite difficult to control
above a freshly blasted area and this problem has been solved by leaving 0.6 m coal at roof.
Venkateswarlu and Raju (1993) stated that roof stability is a function of several factors such
as the inherent physico-mechanical character of the rock, presence of geological anomalies,
method of working and the mining environment and design of roof supports in coalmines
based on geomechanical classificatory.
Raju (1986) observed that first main fall took place after an area of exposure of 6600 sq.m in
goaf. Subsequently the main falls took palace regularly after every 5310 sq.m to 11000 sq.m
of area of exposes. At no time during the 18 months extraction period the support system
(roof bolts, channels and props), used in conjunction with LHDs had given any untoward
experience.
2.7 International Scenario
Khair and Peng (1985) expressed their views that it is very unlikely to have pillar failure due
to vertical stress alone. The major factors probably were the rupture of the roof due to abrupt
change in the topography of the coal seam and possible high horizontal stress.
10
From the studies conducted by Wen-Xiu L, Lan-Fang D, Xiao-Bing, H and Wen L (2007) the
prediction of ground surface movements was found to be important problem in rock and soil
mechanics in the excavation activities. Based on results of the statistical analysis of a large
amount of measured data in underground excavation engineering, the fuzzy genetic
programming method (FGPM) of ground surface movements is given by using the theory of
fuzzy probability measures and genetic programming (GP).
Unver and Yasitli (2006) stated that top coal, caving behind the face is the key factor
affecting the efficiency of production at thick coal seams. Their results included that in order
to decrease dilution and increase extraction ratio and production efficiency, the top coal
should be as uniformly fractured as much as possible. Hence, an efficient and continuous coal
flowing behind the face can be maintained. A special pre-fracture blasting strategy just
sufficient enough to form cracks in the top coal is suggested by means of comparing results
from numerical modelling.
Jialin Xu and Minggao (2005) observed that rock strata move upward from the coal seam to
the surface by groups and the breakage and movement of key stratum determine the dynamic
process of rock strata movement.
Cox (2003) suggested that the ground forces generated by a properly installed and tensioned
mine roof truss assembly can provide permanent mine roof support, even in severe ground
conditions. This can be accomplished either by direct suspension of the rock loads within the
potential failure zone above the mine opening or by indirect reinforcement of the natural rock
arch that tends to form within the immediate mine roof.
Tekook and Keune (1999) declared that in Indian deposits with shallow depths and thick
sandstone in the roof, strata control has the same importance as in deep mining. They have
stressed on measurements and observations in galleries and faces, inference of behaviour of
support and strata, verification of the planning and developing the prediction methods.
Garratt (1999) stated that the stresses acting on underground workings are pre-mining
stresses, interaction induced stresses caused by nearby workings and stresses caused by
current excavation.
11
CHAPTER 3
NUMERICAL MODELLING
12
3. NUMERICAL MODELLING
3.1 Overview
FLAC16
is a two-dimensional explicit finite difference program for engineering mechanics
computation. This program simulates the behavior of structures built of soil, rock or other
materials that may undergo plastic flow when their yield limits are reached. Materials are
represented by elements, or zones, which form a grid that is adjusted by the user to fit the
shape of the object to be modeled. Each element behaves according to a prescribed linear or
nonlinear stress/strain law in response to the applied forces or boundary restraints. The
material can yield and flow and the grid can deform and move with the material that is
represented. The explicit, Lagrangian calculation scheme and the mixed-discretization zoning
technique used in FLAC ensure that plastic collapse and flow are modeled very accurately.
Because no matrices are formed, large two-dimensional calculations can be made without
excessive memory requirements. The drawbacks of the explicit formulation are overcome to
some extent by automatic inertia scaling and automatic damping that do not influence the
mode of failure.
Though FLAC was originally developed for geotechnical and mining engineers, the program
offers a wide range of capabilities to solve complex problems in mechanics. Several built-in
constitutive models that permit the simulation of highly nonlinear, irreversible response
representative of geologic, or similar, materials are available. In addition, FLAC contains
many special features including:
Interface elements to simulate distinct planes along which slip and/or separation can
occur
Plane-strain, plane-stress and axisymmetric geometry modes
Groundwater and consolidation models with automatic phreatic surface calculation
Structural element models to simulate structural support
Extensive facility for generating plots of virtually any problem variable
Optional dynamic analysis capability
Optional viscoelastic and viscoplastic models
Optional thermal and thermal coupling to mechanical stress and pore pressure
modeling capability
Optional two-phase flow model to simulate the flow of two immiscible fluids through
a porous medium
13
3.2 Problem Solving With FLAC
The problem is solved by using FLAC in the following sequence of steps :
Grid generation
Boundary and initial conditions
Loading and sequential modeling
Choice of constitutive model and material properties
Ways to improve modeling efficiency
Interpretation of results
3.3 Recommended Steps For Numerical Analysis In Geomechanics
The recommended steps for solving a real life situation can be modelled as follows:
Step 1: Define the objectives for the model analysis
Step 2: Create a conceptual picture of the physical system
Step 3: Construct and run simple idealized models
Step 4: Assemble problem-specific data
Step 5: Prepare a series of detailed model runs
Step 6: Perform the model calculations
Step 7: Present results for interpretation
3.4 Steps for Numerical Modelling
Step 1: Define the Objectives for the Model Analysis
The level of detail to be included in a model often depends on the purpose of the analysis. For
example, if the objective is to decide between two conflicting mechanisms that are proposed
to explain the behavior of a system, then a crude model may be constructed, provided that it
allows the mechanisms to occur. It is tempting to include complexity in a model just because
it exists in reality. However, complicating features should be omitted if they are likely to
have little influence on the response of the model, or if they are irrelevant to the model’s
purpose.
Step 2: Create a Conceptual Picture of the Physical System
It is important to have a conceptual picture of the problem to provide an initial estimate of the
expected behavior under the imposed conditions. Several questions should be asked when
preparing this picture. Considerations will dictate the gross characteristics of the numerical
model, such as the design of the model geometry, the types of material models, the boundary
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conditions, and the initial equilibrium state for the analysis. They will determine whether a
three-dimensional model is required, or if a two-dimensional model can be used to take
advantage of geometric conditions in the physical system.
Step 3: Construct and Run Simple Idealized Models
When idealizing a physical system for numerical analysis, it is more efficient to construct and
run simple test models first, before building the detailed model. Simple models should be
created at the earliest possible stage in a project to generate both data and understanding. The
results can provide further insight into the conceptual picture of the system. Step 2 may need
to be repeated after simple models are run. Simple models can reveal shortcomings that can
be remedied before any significant effort is invested in the analysis.
Step 4: Assemble Problem-Specific Data
The types of data required for a model analysis include:
• Details of the geometry (e.g., profile of underground openings, surface topography, dam